Semiconductor structures using a group III-nitride quaternary material system with reduced phase separation and method of fabrication

ABSTRACT

A group III-nitride quatenary material system and method is disclosed for use in semiconductor structures, including laser diodes, transistors, and photodetectors, which reduces or eliminates phase separation and provides increased emission efficiency and reliability. In an exemplary embodiment the semiconductor structure includes first GaAINAs layer of a first conduction type formed substantially without phase separation, an GaAINAs active layer substantially without phase separation, and a third GaAINAs layer of an opposite conduction type formed substantially without phase separation.

RELATED APPLICATIONS

This application relates to U.S. patent application Ser. No. 09/277,319filed Mar. 26, 1999 in the names of the same inventors and assigned tothe same assignee.

FIELD OF THE INVENTION

This application relates to semiconductor structures and processes, andparticularly relates to group III-nitride materials systems and methodssuch as might be used in blue or ultraviolet laser diodes and othersimilar semiconductors.

BACKGROUND OF THE INVENTION

The development of the blue laser light source has heralded the nextgeneration of high density optical devices, including disc memories,DVDS, and so on. FIG. 1 shows a cross sectional illustration of a priorart semiconductor laser devices. (S. Nakamura, MRS BULLETIN, Vol. 23,No. 5, pp. 37-43, 1998.) On a sapphire substrate 5, a gallium nitride(GaN) buffer layer 10 is formed, followed by an n-type GaN layer 15, anda 0.1 μm thick silicon dioxide (SiO₂) layer 20 which is patterned toform 4 μm wide stripe windows 25 with a periodicity of 12 μm in theGaN<1-100>direction. Thereafter, an n-type GaN layer 30, an n-typeindium gallium nitride (In_(0.1)Ga_(0.9)N) layer 35, an n-type aluminumgallium nitride (Al_(0.14)Ga_(0.86) N)/GaN MD-SLS (Modulation DopedStrained-Layer Superlattices) cladding layer 40, and an n-type GaNcladding layer 45 are formed. Next, anIn_(0.02)Ga_(0.98)N/In_(0.15)Ga_(0.85)N MQW (Multiple Quantum Well)active layer 50 is formed followed by a p-type Al_(0.2)Ga_(0.8)Ncladding layer 55, a p-type GaN cladding layer 60, a p-typeAl_(0.14)Ga_(0.86)N/GaN MD-SLS cladding layer 65, and a p-type GaNcladding layer 70. A ridge stripe structure is formed in the p-typeAl_(0.14)Ga_(0.86)N/GaN MD-SLS cladding layer 65 to confine the opticalfield which propagates in the ridge waveguide structure in the lateraldirection. Electrodes are formed on the p-type GaN cladding layer 70 andn-type GaN cladding layer 30 to provide current injection.

In the structure shown in FIG. 1, the n-type GaN cladding layer 45 andthe p-type GaN 60 cladding layer are light-guiding layers. The n-typeAl_(0.14)Ga_(0.86)N/GaN MD-SLS cladding layer 40 and the p-typeAl_(0.14)Ga_(0.86)N/GaN MD-SLS cladding layer 65 act as cladding layersfor confinement of the carriers and the light emitted from the activeregion of the InGaN MQW layer 50. The n-type In_(0.1)Ga_(0.9)N layer 35serves as a buffer layer for the thick AlGaN film growth to preventcracking.

By using the structure shown in FIG. 1, carriers are injected into theInGaN MQW active layer 50 through the electrodes, leading to emission oflight in the wavelength region of 400 nm. The optical field is confinedin the active layer in the lateral direction due to the ridge waveguidestructure formed in the p-type Al_(0.14)Ga_(0.86)N/GaN MD-SLS claddinglayer 65 because the effective refractive index under the ridge striperegion is larger than that outside the ridge stripe region. On the otherhand, the optical field is confined in the active layer in thetransverse direction by the n-type GaN cladding layer 45, the n-typeAl_(0.14)Ga_(0.86)N/GaN MD-SLS cladding layers 40, the p-type GaNcladding layer 60, and the p-type Al_(0.14)Ga_(0.86)N/GaN MD-SLScladding layer 65 because the refractive index of the of the activelayer is larger than that of the n-type GaN cladding layer 45 and thep-type GaN cladding layer 60, the n-type Al_(0.14)Ga_(0.86)N/GaN MD-SLSlayer 40, and the p-type GaN cladding layer 60. Therefore, fundamentaltransverse mode operation is obtained.

However, for the structure shown in FIG. 1, it is difficult to reducethe defect density to the order of less than 10⁸ cm⁻², because thelattice constants of AlGaN, InGaN, and GaN are sufficiently differentfrom each other that defects are generated in the structure as a way torelease the strain energy whenever the total thickness of the n-typeIn_(0.1)Ga_(0.9)N layer 35, the In_(0.02)Ga_(0.98)N/In_(0.15)Ga_(0.85)NMQW active layer 50, the n-type Al_(0.14)Ga_(0.86)N/GaN MD-SLS claddinglayer 40, the p-type Al_(0.14)Ga_(0.86)N/GaN MD-SLS cladding layer 65,and the p-type Al_(0.2)Ga_(0.8)N cladding layer 55 exceeds the criticalthickness. The defects result from phase separation and act asabsorption centers for the lasing light, causing decreased lightemission efficiency and increased threshold current. The result is thatthe operating current becomes large, which in turn causes reliability tosuffer.

Moreover, the ternary alloy system of InGaN is used as an active layerin the structure shown in FIG. 1. In this case, the band gap energychanges from 1.9 eV for InN to 3.5 eV for GaN. Therefore, ultravioletlight which has an energy level higher than 3.5 eV cannot be obtained byusing an InGaN active layer. This presents difficulties, sinceultraviolet light is attractive as a light source for the optical pickup device in, for example, higher density optical disc memory systemsand other devices.

To better understand the defects which result from phase separation inconventional ternary materials systems, the mismatch of latticeconstants between InN, GaN, and AlN must be understood. The latticemismatch between InN and GaN, between InN and AlN, and between GaN andAlN, are 11.3%, 13.9%, and 2.3%, respectively. Therefore, an internalstrain energy accumulates in an InGaAlN layer, even if the equivalentlattice constant is the same as that of the substrate due to the factthat equivalent bond lengths are different from each other between InN,GaN, and AlN. In order to reduce the internal strain energy, there is acompositional range which phase separates in the InGaAlN latticemismatched material system, where In atoms, Ga atoms, and Al atoms areinhomogeneously distributed in the layer. The result of phase separationis that In atoms, Ga atoms, and Al atoms in the InGaAlN layers are notdistributed uniformly according to the atomic mole fraction in eachconstituent layer. In turn, this means the band gap energy distributionof any layer which includes phase separation also becomes inhomogeneous.The band gap region of the phase separated portion actsdisproportionately as an optical absorption center or causes opticalscattering for the waveguided light. As noted above, a typical prior artsolution to these problems has been to increase drive current, thusreducing the life of the semiconductor device.

As a result, there has been a long felt need for a semiconductorstructure which minimizes lattice defects due to phase separation andcan be used, for example, as a laser diode which emits blue or UV lightat high efficiency, and for other semiconductor structures such astransistors.

SUMMARY OF THE INVENTION

The present invention substantially overcomes the limitations of theprior art by providing a semiconductor structure which substantiallyreduces defect densities by materially reducing phase separation betweenthe layers of the structure. This in turn permits substantially improvedemission efficiency.

To reduce phase separation, it has been found possible to provide asemiconductor device with GaAlNAs layers having homogeneous Al contentdistribution as well as homogeneous As content distribution in eachlayer. In a light emitting device, this permits optical absorption lossand waveguide scattering loss to be reduced, resulting in a highefficiency light emitting device. By carefully selecting the amounts ofAl and As, devices with at least two general ranges of band gaps may beproduced, allowing development of light emitting devices in both theinfrared and the blue/uv ranges.

In a first exemplary embodiment of a GaAlNAs quaternary material systemin accordance with the present invention, sufficient homogeneity toavoid phase separation has been found when the Al content, representedby x, and the As content, represented by y, ideally satisfy thecondition that 3.18(1−x)(1−y)+3.99(1−x)y+3.11x(1−y)+4xy nearly equals toa constant value. In a typical embodiment of a light emitting device,the constant value may be 3.18. The lack of phase separation resultsbecause the lattice constants of the constituent layers in the structureare sufficiently close to each, in most cases being nearly equal, thatthe generation of defects is suppressed.

A device according to the present invention typically includes a firstlayer of GaAlNAs material of a first conductivity, an GaAlNAs activelayer, and a layer of GaAlNAs material of an opposite conductivitysuccessively formed on one another. By maintaining the mole fractionsessentially in accordance with the formula3.18(1−x)(1−y)+3.99(1−x)y+3.11x(1−y)+4xy nearly equals to a constantvalue, for example on the order of or nearly equal to 3.18, the latticeconstants of the constituent layers remain substantially equal to eachother, leading to decreased generation of defects.

In an alternative embodiment, the semiconductor structure is fabricatedessentially as above, using a quaternary materials system to eliminatephase separation and promote homogeneity across the layer boundaries.Thus, as before, the first cladding layer is a first conduction type andcomposition of GaAlNAs, the active layer is GaAlNAs of a secondcomposition, and the second cladding layer is an opposite conductiontype of GaAlNAs having the composition of the first layer. However, inaddition, the second cladding layer has a ridge structure. As before,the optical absorption loss and waveguide scattering loss is reduced,leading to higher efficiencies, with added benefit that the opticalfield is able to be confined in the lateral direction in the activelayer under the ridge structure. This structure also permits fundamentaltransverse mode operation.

In a third embodiment of the invention, suited particularly toimplementation as a laser diode, the semiconductor structure comprises afirst cladding of a first conduction type of anGa_(1−x1)Al_(x1)N_(1−y1)As_(y1) material, an active layer of anGa_(1−x2)Al_(x2)N_(1−y2)As_(y2) material, and a second cladding layer ofan opposite conduction type of an Ga_(1−x3)Al_(x3)N_(1−y3)As_(y3)material, each successively formed on the prior layer. In such amaterials system, x1, x2, and x3 define the Al content and y1, y2, andy3 define the As content. Moreover, x1, y1, x2, y2, x3, and y3 have arelationship of 0<x1<1, 0<x2<1, 0<x3<1, 0<y1<1, 0<y2<1, 0<y3<1,0.26x1+37y1<=1, 0.26x2+37y2<=1, 0.26x3+37y3<=1,Eg_(GaN)(1−x1)(1−y1)+Eg_(GaAs)(1−x1)y1+Eg_(AlN)x1(1−y1)+Eg_(AlAs)x1y1>Eg_(GaN)(1−x2)(1−y2)+Eg_(GaAs)(1−x2)y2+Eg_(AlN)x2(1−y2)+Eg_(AlAs)x2y2,andEg_(GaN)(1−x3)(1−y3)+Eg_(GaAs)(1−x3)y3+Eg_(AlN)x3(1−y3)+Eg_(AlAs)x3y3>Eg_(GaN)(1−x2)(1−y2)+Eg_(GaAs)(1−x²)y2+Eg_(AlN)x2(1−y2)+Eg_(AlAs)x2y2,where Eg_(GaN), Eg_(GaAs), Eg_(AlN), and Eg_(AlAs) are the band gapenergy of GaN, GaAs, AlN, and AlAs, respectively.

To provide a reproducible semiconductor structure according to the abovematerials system, an exemplary embodiment of GaAlNAs layers have Alcontent, x, and As content, y, which satisfy the relationship 0<x<1,0<y<1 and 0.26x+37y<=1. As before, this materials system permitsreduction of the optical absorption loss and the waveguide scatteringloss, resulting in a high efficiency light emitting device. Moreover,the band gap energy of the GaAlNAs of an active layer becomes smallerthan that of the first cladding layer and the second cladding layer whenx1, y1, x2, y2, x3, and y3 have a relationship of 0<x1<1, 0<x2<1,0<x3<1, 0<y1<1, 0<y2<1, 0<y3<1, 0.26x1+37y1<=1, 0.26x2+37y2<=1,0.26x3+37y3<=1, Eg_(GaN)(1−x1)(1−y1)+Eg_(GaAs)(1−x1)y1+Eg_(AlN)x1(1−y1)+Eg_(AlAs)x1y1>Eg_(GaN)(1−x2)(1−y2)+Eg_(GaAs)(1−x2)y2+Eg_(AlN)x2(1−y2)+Eg_(AlAs)x2y2,andEg_(GaN)(1−x3)(1−y3)+Eg_(GaAs)(1−x3)y3+Eg_(AlN)x3(1−y3)+Eg_(AlAs)x3y3>Eg_(GaN)(1−x2)(1−y2)+Eg_(GaAs)(1−x2)y2+Eg_(AlN)x2(1−y2)+Eg_(AlAs)x2y2.Under these conditions, the injected carriers are confined in the activelayer. In at least some embodiments, it is preferable that the thirdlight emitting device has a GaAlNAs single or multiple quantum wellactive layer whose Al content, xw, and As content, yw, of all theconstituent layers satisfy the relationship of 0<xw<1, 0<yw<1, and0.26xw+37yw<=1.

One of the benefits of the foregoing structure is to reduce thethreshold current density of a laser diode. This can be achieved by useof a single or multiple quantum well structure, which reduces thedensity of the states of the active layer. This causes the carrierdensity necessary for population inversion to become smaller, leading toa reduced or low threshold current density laser diode.

It is preferred that in the third light emitting device, the conditionof 3.18(1−xs)(1−ys)+3.99(1−xs)ys+3.11xs(1−ys)+4xsys nearly equals to aconstant value—on the order of or near 3.18—is satisfied, wherein xs andys are the Al content and the As content, respectively, in each theconstituent layers. As before, this causes the lattice constants of theeach constituent layers to be nearly equal to each other, which in turnsubstantially minimizes defects due to phase separation.

In a fourth embodiment of the present invention, the semiconductorstructure may comprise a first cladding layer of a first conduction typeof a material Ga_(1−x1)Al_(x1)N_(1−y1)As_(y1), anGa_(1−x2)Al_(x2)N_(1−y2)As_(y2) active layer, and a second claddinglayer of an opposite conduction type of a materialGa_(1−x3)Al_(x3)N_(1−y3)As_(y3), each successively formed one upon theprior layer. In addition, the second cladding layer has a ridgestructure. For the foregoing materials system, x1, x2, and x3 define theAl content, y1, y2, and y3 define the As content, and x1, y1, x2, y2,x3, and y3 have a relationship of 0<x1<1, 0<x2<1, 0<x3<1, 0<y1<1,0<y2<1, 0<y3<1, 0.26x1+37y1<=1, 0.26x2+37y2<=1, 0.26x3+37y3<=1,Eg_(GaN)(1−x1)(1−y1)+Eg_(GaAs)(1−x1)y1+Eg_(AlN)x1(1−y1)+Eg_(AlAs)x1y1>Eg_(GaN)(1−x2)(1−y2)+Eg_(GaAs)(1−x2)y2+Eg_(AlN)x2(1−y2)+Eg_(AlAs)x2y2,andEg_(GaN)(1−x3)(1−y3)+Eg_(GaAs)(1−x3)y3+Eg_(AlN)x3(1−y3)+Eg_(AlAs)x3y3>Eg_(GaN)(1−x2)(1−y2)+Eg_(GaAs)(1−x2)y2+Eg_(AlN)x2(1−y2)+Eg_(AlAs)x2y2,where Eg_(GaN), Eg_(GaAs), Eg_(AlN), and Eg_(AlAs) are the band gapenergy of GaN, GaAs, AlN, and AlAs, respectively.

As with the prior embodiments, each of the GaAlNAs layers have ahomogeneous Al and As content distribution, which can be obtainedreproducibly when Al content, x, and As content, y, of each GaAlNAslayer satisfies the relationship 0<x<1, 0<y<1 and 0.26x+37y<=1. The bandgap energy of the GaAlNAs active layer becomes smaller than that of thefirst cladding layer and the second cladding layer when x1, y1, x2, y2,x3, and y3 have a relationship of x1, y1, x2, y2, x3, and y3 have arelationship of 0<x1<1, 0<x2<1, 0<x3<1, 0<y1<1, 0<y2<1, 0<y3<1,0.26x1+37y1<=1, 0.26x2+37y2<=1, 0.26x3+37y3<=1,Eg_(GaN)(1−x1)(1−y1)+Eg_(GaAs)(1−x1)y1+Eg_(AlN)x1(1−y1)+Eg_(AlAs)x1y1>Eg_(GaN)(1−x2)(1−y2)+Eg_(GaAs)(1−x2)y2+Eg_(AlN)x2(1−y2)+Eg_(AlAs)x2y2,andEg_(GaN)(1−x3)(1−y3)+Eg_(GaAs)(1−x3)y3+Eg_(AlN)x3(1−y3)+Eg_(AlAs)x3y3>Eg_(GaN)(1−x2)(1−y2)+Eg_(GaAs)(1−x2)y2+Eg_(AlN)x2(1−y2)+Eg_(AlAs)x2y2.Similar to the prior embodiments, the injected carriers are confined inthe active layer and the optical field is confined in the lateraldirection in the active layer under the ridge structure, producing afundamental transverse mode operation.

Also similar to the prior embodiments, the fourth embodiment typicallyincludes an GaAlNAs single or multiple quantum well active layer whoseAl content, xw, and As content, yw, of all the constituent layerssatisfy the relationship of 0<xw<1, 0<yw<1, and 0.26xw+37yw<=1. Also,the condition 3.18(1−xs)(1−ys)+3.99(1−xs)ys+3.11xs(1−ys)+4xsys nearlyequals to a constant value, for example on the order of or near 3.18 istypically satisfied, where xs and ys are the Al content and the Ascontent, respectively, in each constituent layer. Similar parametersapply for other substrates, such as sapphire, silicon carbide, and soon.

The foregoing results may be achieved with conventional processingtemperatures and times, typically in the range of 500° C. to 1000° C.See “Growth of high optical and electrical quality GaN layers usinglow-pressure metalorganic chemical vapor deposition,” Appl. Phys. Lett.58 (5), Feb. 4, 1991 p. 526 et seq.

The present invention may be better appreciated by the followingDetailed Description of the Invention, taken together with the attachedFigures.

FIGURES

FIG. 1 shows a prior art laser diode structure using a conventionalternary materials system.

FIG. 2 shows in cross-sectional view a semiconductor structure accordingto a first embodiment of the invention.

FIG. 3 shows a graph of the light-current characteristics of a laserdiode according to the structure of FIG. 1.

FIG. 4 shows an exemplary series of the fabrication steps for asemiconductor structure in accordance with a first embodiment of theinvention.

FIG. 5 shows in cross-sectional view a semiconductor structure accordingto a second embodiment.

FIG. 6 shows a graph of the light-current characteristics of a laserdiode according to the structure of FIG. 5.

FIG. 7 shows an exemplary series of the fabrication steps for asemiconductor structure in accordance with the second embodiment of theinvention.

FIG. 8 is a cross-sectional illustration of a semiconductor laser diodeof the third embodiment.

FIG. 9 shows the light-current characteristics of the laser diode of thethird embodiment.

FIG. 10 shows a series of the fabrication steps of a semiconductor laserdiode in one experiment example of the third embodiment.

FIG. 11 is a cross-sectional illustration of a semiconductor laser diodeof the fourth embodiment.

FIG. 12 shows the light-current characteristics of the laser diode ofthe fourth embodiment.

FIG. 13 shows a series of the fabrication steps of a semiconductor laserdiode in one experiment example of the fourth embodiment.

FIG. 14 shows in plot form the boundary between the phase separationregion and the region without phase separation at various growthtemperatures.

FIG. 15 shows the content choice region of Ga content and Al content inInGaAlN to avoid phase separation at a growth temperature belowapproximately 1000° C.

FIG. 16 shows the content choice line of Ga content and Al content inInGaAlN to avoid phase separation at a growth temperature belowapproximately 1000° C. which, at the same time, creates a latticeconstant of InGaAlN substantially equivalent to that of GaN.

FIGS. 17A and 17B show representations of bipolar and FET transistorsconstructed in accordance with the materials system of the presentinvention.

FIG. 18 shows an implementation of the presention invention as aphototransistor.

FIG. 19 shows an implementation of the present invention as aphotodiode.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 2, shown therein in cross-sectional view is asemiconductor structure according to a first embodiment of theinvention. For purposes of illustration, the semicoductor structureshown in many of the Figures will be a laser diode, although the presentinvention has appliction to a number of device types. With particularreference to FIG. 2, an n-type GaN substrate 100 is provided and ann-type GaN first cladding layer 105 (typically 0.5 μm thick) is formedthereon. Thereafter, a second cladding layer 110, typically of an n-typeGa_(0.75)Al_(0.25)N_(0.979)As_(0.021) material which may be on the orderof 1.5 μm thick, is formed thereon, followed by a multiple quantum wellactive layer 115 which in an exemplary arrangement may comprise threequantum well layers of Ga_(0.95)Al_(0.05)N_(0.996)As_(0.004) material onthe order of 35 Å thick together with four barrier layers of/Ga_(0.85)Al_(0.15)N_(0.987)As_(0.013) material on the order of 35 Åthick, arranged as three pairs. Next, a third cladding layer 120 of ap-type Ga_(0.75)Al_(0.25)N_(0.979)As_(0.021) (typically on the order of1.5 μm thick) is formed, followed by a p-type GaN fourth cladding layer125 (on the order of 0.5 μm thick). A SiO₂ layer 130 having one stripelike window region 135 (3.0 μm width) is formed on the p-type GaN fourthcladding layer 125. A first electrode 140 is formed on the n-type GaNsubstrate 100, while a second electrode 145 is formed on the SiO₂ layer130 and the window region 135.

In order to emit ultra violet light with a wavelength range of 360 nmfrom the active layer 115, the Al content, x, and the As content, y, ofall the layers generally satisfies the condition3.18(1−x)(1−y)+3.99(1−x)y+3.11x(1−y)+4xy nearly equals to a constantvalue, which may be on the order of 3.18 for at least the firstembodiment. To avoid defects due to phase separation, the latticeconstants of the various constituent layers are matched to each other bysetting the Al content, x, and the As content, y, in each of the layersto meet the condition 3.18(1−x)(1−y)+3.99(1−x)y+3.11x(1−y)+4xy nearlyequals to a constant value, again, for the first embodiment on the orderof 3.18±0.05 so that the equivalent lattice constants of each layersbecome nearly equal to the lattice constant of GaN.

By proper selection of materials, the band gap energy of the n-typesecond cladding layer 110 and the p-type third cladding layer 120 arelarger than that of the 3 pairs of multiple quantum well active layers115. This confines the injected carriers from the n-type second claddinglayer 110 and p-type third cladding layer 120 within the active layer115, where the carriers recombine to lead to the emission of ultravioletlight. In addition, the refractive index of the n-type second claddinglayer 110 and the p-type third cladding layer 120 are smaller than thatof the multiple quantum well active layer 115, which confines theoptical field in the transverse direction.

Because the injected current from the electrode 145 is confined to flowthrough the window region 135, the region in the active layer 115 underthe widow region 135 is activated strongly. This causes the local modalgain in the active layer under the window region 135 to be higher thanthe local modal gain in the active layer under the SiO₂ layer 130.Therefore, a gain guided waveguide mechanism, leading to a lasingoscillation, is able to be formed in the structure of the firstembodiment.

FIG. 3 shows a plot of the emitted light versus drive current for alaser diode constructed in accordance with the first embodiment as shownin FIG. 2. The laser diode is driven with a pulsed current with a dutycycle of 1%. The threshold current density is found to be 6.0 kA/cm².

FIGS. 4A-4D show, in sequence, a summary of the fabrication stepsnecessary to construct an exemplary laser diode according to the firstembodiment. Since the structure which results from FIGS. 4A-4D willresemble that shown in FIG. 2, like reference numerals will be used forelements whenever possible. With reference first to FIG. 4A, an n-typeGaN substrate 100 is provided, on which is grown an n-type GaN firstcladding layer 105. The first cladding layer 105 is typically on theorder of 0.5 μm thick. Thereafter, an n-typeGa_(0.75)Al_(0.25)N_(0.979)As_(0.021) second cladding layer 110 isformed, typically on the order of 1.5 μm thick.

Next, a multiple quantum well active layer 115 is formed by creatingthree quantum wells comprised of three layers ofGa_(0.95)Al_(0.05)N_(0.996)As_(0.004) material each on the order of 35 Åthick, together with four barrier layers ofGa_(0.85)Al_(0.15)N_(0.987)As_(0.013) material on the order of 35 Åthick. A third cladding layer 120 of p-typeGa_(0.75)Al_(0.25)N_(0.979)As_(0.021) material, on the order of 1.5 μmthick, is then formed, after which is formed a fourth cladding layer 125of a p-type GaN on the order of 0.5 μm thick. Each of the layers istypically formed by either the Metal Organic Chemical Vapor Deposition(MOCVD) method or the Molecular Beam Epitaxy (MBE) method.

Then, as shown in FIG. 4B, a silicon dioxide (SiO₂) layer 130 is formedon the p-type GaN fourth cladding layer 125, for example by the ChemicalVapor Deposition (CVD) method. Using photolithography and etching or anyother suitable method, a window region 135 is formed as shown in FIG.4C. The window region 135 may be stripe-like in at least someembodiments. Finally, as shown in FIG. 4D, a first electrode 140 and asecond electrode 145 are formed on the n-type GaN substrate 100 and onthe SiO₂ layer 130, respectively, by evaporation or any other suitableprocess.

Referring next to FIG. 5, a second embodiment of a semiconductorstructure in accordance with the present invention may be betterappreciated. As with the first embodiment, an exemplary application ofthe second embodiment is the creation of a laser diode. The structure ofthe second embodiment permits a waveguide mechanism to be built into thestructure with a real refractive index guide. This provides a lowthreshold current laser diode which can operate with a fundamentaltransverse mode.

Continuing with reference to FIG. 5, for ease of reference, likeelements will be indicated with like reference numerals. On an n-typeGaN substrate 100, a first cladding layer 105 is formed of an n-type GaNon the order of 0.5 μm thick. Successively, an n-type second claddinglayer 110 is formed of Ga_(0.75)Al_(0.25)N_(0.979)As_(0.021) material onthe order of 1.5 μm thick. Thereafter, a multiple quantum well activelayer 115 is formed comprising three well layers ofGa_(0.95)Al_(0.005)N_(0.996)As_(0.004) material on the order of 35 thicktogether with four barrier layers ofGa_(0.85)Al_(0.15)N_(0.987)As_(0.013) material, also on the order of 35Å thick. Next, a third, p-type cladding layer 120 formed ofGa_(0.75)Al_(0.25)N_(0.979)As_(0.021) material on the order of 1.5 μmthick is formed. Thereafter, a p-type GaN fourth cladding layer 125 onthe order of 0.5 μm thick is formed over the third cladding layer 120.The third and fourth cladding layers are then partially removed tocreate a ridge structure. A silicon dioxide (SiO₂) layer 130 is thenformed over the fourth cladding layer 125 as well as the remainingexposed portion of the third cladding layer 120. A window region 135,which may be stripe-like on the order of 2.0 μm width, is formed throughthe SiO₂ layer above the fourth and third cladding layers 125 and 120,respectively. As with the first embodiment, a first electrode 140 isformed on the n-type GaN substrate 100 and a second electrode 145 isformed on the SiO₂ layer 130 and the window region 135.

As with the first embodiment, in order to emit violet light with awavelength in the range of 350 nm from the active layer 115, the Alcontent, x, and the As content, y, of the well layer is set to be 0.05,0.004, respectively. Likewise, in order to match the lattice constantsof each of the constituent layers to avoid defects due to strain, the Alcontent, x, and the As content, y, of all the layers satisfies thecondition 3.18(1−x)(1−y)+3.99(1−x)y+3.11x(1−y)+4xy nearly equals to aconstant value, which may for example be 3.18±0.05. Likewise, the bandgap energy of the cladding layers is maintained larger than the band gapenergy for the active layer, allowing the emission of violet light.Similarly the refractive index of the materials is as discussed inconnection with the first embodiment, permitting the optical field to beconfined in the transverse direction.

Similar to the operation of the first embodiment, the region of theactive layer 115 under the window region 135 is activated stronglybecause of the constraints on the injected current by the SiO₂ layer.The result, again, is that the local modal gain in the active layerunder the window region 135 is higher than the local modal gain in theactive layer under the SiO₂ layer 130. This, combined with therelatively higher effective refractive index in the transverse directioninside the ridge stripe region compared to that outside the ridge striperegion, provides an effective refractive index step (Δn). This resultsin a structure which has, built in, a waveguide mechanism of a realrefractive index guide. Therefore, the design of the second embodimentprovides a low threshold current laser diode which can operate with afundamental transverse mode.

FIG. 6 shows in graph form the emitted light versus drive currentcharacteristics of a laser diode in accordance with the secondembodiment. The laser diode is driven with a cw current. The thresholdcurrent is found to be 38.5 mA.

Referring next to FIGS. 7A-7E, a summary of the key fabrication steps isshown for an exemplary device of a semiconductor laser diode inaccordance with the second embodiment.

Referring first to FIGS. 7A and 7B, the formation of the first andsecond cladding layers 105 and 110 on an n-type GaN substrate 100,together with the three-pair multiple quantum well active layer 115 arethe same as for the first embodiment. Thereafter, the third and fourthcladding layers 120 and 125 are formed and then partiallyremoved—typically by etching—to create a ridge structure. As before, inan exemplary embodiment the various layers are formed successively byeither the MOCVD or the MBE method.

Then, as shown in FIGS. 7C-7E, a silicon dioxide layer 130 is formedover the fourth and third cladding layers 125 and 120, respectively,typically by the CVD method, after which a window region 135 is formedas with the first embodiment. Electrodes 140 and 145 are then evaporatedor otherwise bonded to the structure.

Referring next to FIG. 8, a third embodiment of the present inventionmay be better appreciated. The third embodiment provides slightlydifferent mole fractions to permit the emission of ultra violet light,but is otherwise similar to the first embodiment. Thus, an n-type GaNsubstrate 100 continues to be used, together with an n-type GaN firstcladding layer 105. However, the second cladding layer 810 is typicallyof n-type Ga_(0.58)Al_(0.42)N_(0.983)As_(0.017) material on the order of1.5 μm thick, while the three-pair quantum well active layer 815typically includes three barrier layers ofGa_(0.78)Al_(0.22)N_(0.999)As_(0.001) material together with fourbarrier layers of Ga_(0.73)Al_(0.27)N_(0.995)As_(0.005) material. Thethird cladding layer 820 is typically a p-typeGa_(0.58)Al_(0.42)N_(0.983)As_(0.017) material, while the fourthcladding layer 125 is, like the first embodiment, a p-type GaN material.The thicknesses of each layer are substantially the same as for thefirst embodiment. A SiO₂ layer 130, window region 135, and first andsecond electrodes 140 and 145 complete the structure.

In order to emit blue light in a wavelength range of 410 nm from theactive layer 815, the Al content and the As content within the welllayer 815 is set to be 0.22 and 0.001, respectively. In order to matchthe lattice constants of the constituent layers to avoid generation ofstrain-induced defects, the Al content, x, and the As content, y, ofeach of the layers is set to satisfy the condition3.18(1−x)(1−y)+3.99(1−x)y+3.11x(1−y)+4xy nearly equals to a constantvalue. For exemplary purposes of the third embodiment, the constantvalue may be on the order of 3.17±0.05.

Although the third embodiment emits blue light whereas the firstembodiment emits ultraviolet light, the band gap energies of claddinglayers continue to be set higher than the band gap energy of the threepairs of the multiple quantum well active layer 815. As before, thatpermits carrier confinement and recombination in the active layer 815.Also as with the first embodiment, the refractive index of the secondand third cladding layers is, by design, smaller than that of the activelayer, causing the optical field to be confined in the transversedirection. Likewise, the strong current injection under the windowregion 135 yields comparatively higher local modal gain in the activelayer relative to the portion of the active layer under the SiO₂ layer130, again resulting in a guided waveguide mechanism which leads to alasing oscillation.

FIG. 9 shows a plot of the emitted light versus drive currentcharacteristics of the laser diode in accordance with the thirdembodiment. The laser diode is driven with a pulsed current with a dutycycle of 1%. The threshold current density is found to be 5.7 kA/cm².

FIGS. 10A-10D show a series of the fabrication steps of a semiconductorlaser diode in one example of the third embodiment. It will beappreciated that the fabrication steps are the same as those describedin connection with FIGS. 4A-4D, and therefore are not further described.

Referring next to FIG. 11, a fourth embodiment of the present inventionmay be better appreciated. The fourth embodiment, like the thirdembodiment, is designed to emit blue light and therefore has the same Aland As content as the third embodiment. However, like the secondembodiment, the fourth embodiment is configured to provide a ridgestructure to serve as a waveguide. Because the Al and As content issimilar to that of FIG. 8, similar elements will be described with thereference numerals used in FIG. 8.

Continuing to refer to FIG. 11, the structure of the fourth embodimentcan be seen to have a GaN substrate 100 on which is a formed a firstcladding layer 105 followed by a second cladding layer 810. A three-pairmultiple quantum well active layer 815 is formed there above, followedby a third cladding layer 820. A fourth cladding layer 125, silicondioxide layer 130, windows 135 and electrodes 140 and 145 are all formedas before. The materials, including the Al content and As content,remain as shown for FIG. 8, or 0.22 and 0.001, respectively. Likewisethe Al content, x, and the As content, y, of the layers is set tosatisfy the condition 3.18(1−x)(1−y)+3.99(1−x)y+3.11x(1−y)+4xy nearlyequals to a constant value on the order of 3.17, as with the thirdembodiment. The band gap energy, refractive index and modal gain forcurrent injection are all substantially as discussed in connection withthe third embodiment and are not further discussed.

FIG. 12 plots drive current versus emitted light of a laser diodeconstructed in accordance with the fourth embodiment. The laser diode isdriven with a cw current. The threshold current is found to be 33.0 mA.

FIG. 13 shows a summary of the fabrication steps of a semiconductorlaser diode in accordance with the fourth embodiment. The steps areessentially identical to those discussed in connection with FIGS. 7A-7Eand are not further discussed.

Referring next to FIG. 14, the selection of the Al content, x, and theAs content, y, and the relationship therebetween for the constituentGaAlNAs layers may be better understood. In particular, the relative Aland As contents are required to satisfy, approximately, the relationship0<x<1, 0<y<1, 0.26x+37y<=1.

In GaAlNAs material system, the lattice constant of GaN, AlN, GaAs andAlAs are different from each other. For example, the lattice mismatchbetween GaN and GaAs, between AlN and AlAs, and between GaN and AlN, are25.4%, 28.6%, and 2.3%, respectively. Therefore, an internal strainenergy is accumulated in GaAlNAs layer, even if the equivalent latticeconstant is the same as that of the substrate due to the fact thatequivalent bond length are different from each other between GaN, AlN,GaAs and AlAs. FIG. 14 shows the boundary of phase separation regionplotted against various growth temperatures. The lines in FIG. 14 showthe boundary between the compositionally unstable (phase separation)region and stable region with respect to various temperatures. In thosecases where phase separation occurs, Ga atoms, Al atoms, N atoms, and Asatoms in the GaAlNAs layers are not distributed uniformly according tothe atomic content in each constituent layer. Stated differently, theband gap energy distribution of the phase separated layer also becomesinhomogeneous in the layer. The region of the relatively small band gapregion in the phase separated layer acts as an optical absorptioncenter, or causes optical scattering for the waveguided light. Thismeans that the phase separation phenomena should be avoided to obtain ahigh efficiency light emitting device.

Referring still to FIG. 14, it can be seen that the phase separationregion varies with temperature. The lines in FIG. 14 show the boundarybetween the compositionally unstable region—that is, resulting in phaseseparation—and the stable region with respect to various temperatures.The region surrounded with the GaAs-GaN line, AlAs-AlN line and theboundary line shows the phase separation content region. It has beendiscovered that the ternary alloys AlNAs and GaNAs have a large phaseseparation region due to the large lattice mismatch between AlN andAlAs, and between GaN and GaAs. On the other hand, it has been foundthat the ternary alloys GaAlN and GaAlAs have no phase separation regionat crystal growth temperatures around 1000° C., due to the small latticemismatch between AlN and GaN, and between AlAs and GaAs.

It has therefore been discovered that an GaAlNAs material system can beprovided in which the usual crystal growth temperature is in theapproximate range of around 600° C. to around 1000° C. Likewise, it hasbeen discovered that phase separation of the Al content and As contentof GaAlNAs does not occur in significant amounts at processingtemperatures between on the order of 600° C. and on the order of 1000°C. Finally, by combining the two, the content choice region of Alcontent and As content in GaAlNAs to avoid phase separation at a crystalgrowth temperature below around 1000° C. is found to be the shadowregion in FIG. 15, with the line separating the two regions beingapproximately defined by the relationship 0.26x+37y=1.

Therefore, for each of the four structural embodiments disclosedhereinabove, the phase separation phenomena can be avoided in an InGaAlNmaterial system by operating at a crystal growth temperature between onthe order of 600° C. and around 1000° C., when the Ga mole fraction, x,and the AlN mole fraction, y, of the all constituent layers of the laserdiodes are made to satisfy approximately the relationship of 0<x<1,0<y<1, 0.26x+37y<=1. The result is the substantially uniformdistribution of Ga atoms, Al atoms, N atoms and As atoms in eachconstituent layer according to the atomic mole fraction.

FIG. 16 shows the content choice line of Al content, x, and As content,y, in an GaAlNAs system to avoid the phase separation phenomenon atgrowth temperatures below around 1000° C. and still ensure a reasonablelattice match to GaN. The line in FIG. 16 shows the exemplary line of3.18(1−x)(1−y)+3.99(1−x)y+3.11x(1−y)+4xy=3.18. Therefore, by ensuringthat the Al content and As content of the constituent GaAlNAs layers ofa laser diode formed on a GaN substrate have a relationship of3.18(1−x)(1−y)+3.99(1−x)y+3.11x(1−y)+4xy nearly equals to 3.18, 0<x<1,0<y<1, and 0.26x+37y<=1, a laser diode on a GaN substrate with lowdefect density and no or very little phase separation can be obtained.

In addition, other semiconductor structures can be fabricated with thematerials system discussed above. Group-III nitride materials,especially GaN and AlN, are promising for use in electronic deviceswhich can operate under high-power and high-temperature conditions—forexample, microwave power transistors. This results, in part, from theirwide band gap (3.5 eV for GaN and 6.2 eV for AlN), high breakdownelectronic field, and high saturation velocity. By comparison, the bandgaps of AlAs, GaAs, and Si are 2.16 eV, 1.42 eV, and 1.12 eV,respectively. This has led to significant research in the use ofAlGaN/GaN materials for such field effect transistors (FETs). However,as noted previously hereinabove, the different lattice constants ofAlGaN and GaN cause the generation of significant defects, limiting themobility of electrons in the resultant structure and the utility of suchmaterials systems for FET use.

The present invention substantially overcomes these limitations, in thatthe GaAlNAs/GaN material of the present invention has a lattice constantequal to GaN. As discussed hereinabove, a quaternary materials system ofGa_(1−x)Al_(x)N_(1−y)As_(y), where the Al content (x) and As content (y)satisfy the relationships 0<x<1, 0<y<1, 0.26x+37Y<=1,3.18(1−x)(1−y)+3.99(1−x)y+3.11x(1−y)+4xy equals to 3.18, not only has aband gap greater than 3.5 eV, but also has a lattice constantsubstantially equal to GaN. This permits fabrication of semiconductorstructures such as FETs which have substantially uniform atomic contentdistribution in the various layers. Therefore, by using a GaAlNAs/GaNmaterial system in accordance with the present invention, whose Al molefraction, x and As mole fraction, y satisfy the above relationships,high-power and high-temperature transistors with low defect density canbe realized.

Referring to FIG. 17A, there is shown therein an exemplary embodiment ofa heterojunction field effect transistor(HFET) using GaAlNAs/GaNmaterial in accordance with the present invention. On a GaN substrate520, a 0.5 μm i-GaN layer 525 is formed, followed by a thin,approximately 10 nm GaN conducting channel layer 530 and a 10 nm GaAlNAslayer 535. Source and drain electrodes 540A-B, and gate electrode 545are formed in a conventional manner. In the structure, the Al content,x, and As content, y, of the GaAlNAs layer are set to be 0.25 and 0.021,respectively. In this case, the value of x and y satisfy therelationship of 0<x<1, 0<y<1, 0.26x+37Y<=1,3.18(1−x)(1−y)+3.99(1−x)y+3.11x(1−y)+4xy=3.18. This results in anGaAlNAs layer substantially without phase separation and with a latticeconstant equal to GaN, In turn, this permits high electron velocities tobe achieved because the two dimensional electron gas formed in theheterointerface of GaAlNAs and GaN layer is not scattered by anyfluctuation in atomic content of the GaAlNAs layer (such as would becaused in the presence of defects). Moreover, the band gap of theGaAlNAs is larger than 4 eV so that reliable high-temperature operationcan be achieved by using the structure shown in FIG. 17A.

Similarly, FIG. 17B shows an embodiment of a heterojunction bipolartransistor(HBT) in accordance with the present invention. On the GaNsubstrate 550, a 400 nm thick n-type GaAlNAs collector layer 555 isformed, followed by a 50 nm thick p-type GaN base layer 560, and a 300nm thick n-type GaAlNAs emitter layer 565. Base electrode 570, collectorelectrode 575 and emitter electrode 580 are formed conventionally. Aswith FIG. 17A, for the exemplary embodiment of FIG. 17B the Al and Ascontents x and y of the GaAlNAs layer are set to be 0.25, 0.021,respectively, and x and y are required to satisfy the same relationshipsas discussed above. As with FIG. 17A, an GaAlNAs layer withoutsignificant phase separation and with a lattice constant equal to GaN isrealized, resulting in a very high quality heterojunction ofGaAlNAs/GaN. In addition, the band gap of the GaAlNAs emitter layer (4eV) is larger than that of the GaN base layer (3.5 eV) so that holes inthe p-type base layer are well confined in that base layer. This resultsbecause of the larger valence band discontinuity between GaN and GaAlNAsthan would occur in a GaN homojunction bipolar transistor. This has thebenefit of obtaining a large current amplification of collector currentrelative to base current. Moreover, as mentioned above, the bandgap ofthe GaAlNAs and the GaN layer is large so that the transistor can beused reliably in high-temperature applications.

Referring next to FIG. 18, there is shown therein an implementation ofthe present invention as a phototransistor. In this regard, GaN andAlGaN are attractive materials for photo detectors in ultraviolet(UV)range, since both GaN and AlN have a wide band gap (3.5 eV for GaN whichcorresponds to the light wavelength of 350 nm, 6.2 eV for AlN whichcorresponds to the light wavelength of 200 nm). Due to the direct bandgap and the availability of AlGaN in the entire AlN alloy compositionrange, AlGaN/GaN based UV photo detectors have the advantage of highquantum efficiency, as well as tunability of high cut-off wavelength.However, the lattice constant of AlGaN is sufficiently different fromGaN that defects tend to be formed, which leads increased leakagecurrent. Ga_(1−x)Al_(x)N_(1−y)As_(y), where the Al content (x) and Ascontent (y) satisfy the relationships 0<x<1, 0<y<1, 0.26x+37y<=1, offersnot only a band gap larger than 2.8 eV, but also can be fabricated inlayers with equal atomic content distribution, so that GaAlNAs materialalso can be used for UV photo detector applications. Moreover, theGa_(1−x)Al_(x)N_(1−y)As_(y) quaternary material whose Al content, x andAs content, y satisfy the relationship of3.18(1−x)(1−y)+3.99(1−x)y+3.11x(1−y)+4xy=3.18 has a lattice constantequal to GaN and a bandgap larger than 3.5 eV. Therefore, by usingGaAlNAs/GaN material whose Al content, x and As content, y satisfy theabove relationship, UV photo detectors with low defect density can berealized. In the event that detection of other frequencies is desired,for example blue light, only slight modification is required.

As shown in FIG. 18, the semiconductor device of the present inventioncan be implemented as a heterojunction phototransistor(HPT) usingGaAlNAs/GaN material. On the GaN substrate 700, a GaAlNAs collectorlayer 705 is formed on the order of 500 nm thick n-type, followed by theformation of a 200 nm thick p-type GaN base layer 710. Thereafter, aGaAlNAs emitter layer 715 on the order of 500 nm thick is formed. On theemitter layer, a ring shaped electrode 720 is formed to permit light toimpinge on the base layer.

In an exemplary structure, the Al content, x and As contnet, y of theGaAlNAs layer are set to be 0.25 and 0.021, respectively. In this case,the value of x and y satisfy the relationship of 0<x<1, 0<y<1,0.26x+37Y<=1, 3.18(1−x)(1−y)+3.99(1−x)y+3.11x(1−y)+4xy=3.18, so that anGaAlNAs layer can be formed which substantially avoids phase separationwhile having a lattice constant equal to GaN, thus permitting theformation of a high quality heterojunction of GaAlNAs/GaN. The band gapof the GaAlNAs emitter layer (4 eV which corresponds to the lightwavelength of 307 nm) is larger than that of GaN base layer (3.5 eVwhich corresponds to the light wavelength of 350 nm). The light impingeson the emitter side. For the embodiment shown, impinging light in thewavelength range between 307 nm and 350 nm is transparent to the emitterlayer, so that the light in that range is absorbed in the GaN base layerand generates electron and hole pairs. The holes generated by theoptical absorption in the p-type base layer are well confined in thebase layer because the valence band discontinuity between GaN andGaAlNAs is larger than that for a conventional GaN homojunction phototransistor. This leads to the induction of a larger emitter current,which offers better electronic neutralization in the base region than inthe case of the homojunction photo transistor. Therefore, UV photodetectors with high quantum efficiency and high sensitivity, and theresultant high conversion efficiency from input light to collectorcurrent, are obtained. In the event that other frequencies are to bedetected, the GaN base layer may be replaced with, for example for bluelight, InGaN.

In addition to the phototransistor of FIG. 18, it is also possible toimplement a photodiode in accordance with the present invention.Referring to FIG. 19, an n-type substrate 900 is provided, on which isformed an n-type layer 905 of Ga_(1−x)Al_(x)N_(1−y)As_(y) quaternarymaterial or equivalent, which conforms to the relationships discussedabove in connection with FIG. 18. A layer 910 is formed. An active layer915 is thereafter formed, and above that is formed a layer 920 of p-typeGa_(1−x)Al_(x)N_(1−y)As_(y) quaternary material. Then, a p-type secondcladding layer 925 is formed above the layer 920, and a window 930 isformed therein to expose a portion of the layer 920. The window 930provides a port by which light can impinge on the layer 920, causing thecreation of holes. A pair of electrodes 935 and 940 may be fabricated ina conventional manner, with the electrode 935 typically being a ringelectrode around the window 930. It will be appreciated that the bandgap of the second cladding layer 925 is preferably larger than the bandgap of the layer 920, which is in turn preferably larger than the bandgap of the active layer 915; such an approach provides sensitivity tothe widest range of wavelength of light. If the event a narrower rangeis desired, a material with a lower band gap than the layer 920 may beused for the layer 925. In addition, it is also not necessary to includethe layer 925 in all embodiments, as the layers 910, 915 and 920provide, in at least some instances, an adequate photosensitivepn-junction.

Having fully described a preferred embodiment of the invention andvarious alternatives, those skilled in the art will recognize, given theteachings herein, that numerous alternatives and equivalents exist whichdo not depart from the invention. It is therefore intended that theinvention not be limited by the foregoing description, but only by theappended claims.

We claim:
 1. A semiconductor structure comprising: a first claddinglayer of GaAlNAs material having a first conduction type, an GaAlNAsactive layer, and a second cladding layer of GaAlNAs material having aconduction type opposite the first conduction type, wherein the Alcontent, x, and the As content y, of all the constituent layers satisfythe condition that 0.26x+37y<=1.
 2. A light emitting device according toclaim 1, wherein the Al content, x, and the As content, y, of all theconstituent layers satisfy the condition that3.18(1−x)(1−y)+3.99(1−x)y+3.11x(1−y)+4xy nearly equals to a constantvalue.
 3. A light emitting device according to claim 1, wherein the GaNmole fraction, x, and the AlN, y, of all the constituent layers satisfythe condition that 3.18(1−x)(1−y)+3.99(1−x)y+3.11x(1−y)+4xy nearlyequals 3.18.
 4. A semiconductor structure comprising: a first claddinglayer of GaAlNAs material having a first conduction type, an GaAlNAsactive layer, and a second cladding layer of GaAlNAs material having aconduction type opposite the first conduction type, the crystal growthtemperature and the mole fractions of the constituent elements of eachlayer being selected to minimize phase separation, wherein the Alcontent, x, and the As content, y, of all of the constituent layerssatisfy the condition that 0.26x+37y<=1.
 5. A light emitting devicecomprising: a first conduction type of an GaAlNAs first cladding layerwithout phase separation, an GaAlNAs active layer without phaseseparation, and a second conduction type of GaAlNAs second claddinglayer without phase separation, said GaAlNAs second cladding layerhaving a ridge structure, all successively formed one upon each other,wherein the Al content, x, and the As content, y, of all the constituentlayers satisfy the condition that 0.26x+37y<=1.
 6. A light emittingdevice according to claim 5, wherein the Al content, x, and the Ascontent, y, of all the constituent layers satisfy the condition that3.18(1−x)(1−y)+3.99(1−x)y+3.11x(1−y)+4xy nearly equals a constant value.7. A light emitting device according to claim 5, wherein the Al content,x, and the As content, y, of all the constituent layers satisfy thecondition that 3.18(1−x)(1−y)+3.99(1−x)y+3.11x(1−y)+4xy nearly equals31.8.
 8. A light emitting device comprising: a certain conduction typeof a Ga_(1−x1)Al_(x1)N_(1−y1)As_(y1) first cladding layer of saidcertain conduction type, a Ga_(1−x2)Al_(x2)N_(1−y2)As_(y2) active layer,an opposite conduction type of a Ga_(1−x3)Al_(x3)N_(1−y3)As_(y3) secondcladding layer, all successively formed one upon each other, wherein x1,x2, and x3 define the Al content, y1, y2, and y3 define the As content,and x1, y1, x2, y2, x3, and y3 have a relationship of 0<x1<1, 0<x2<1,0<x3<1, 0<y1<1, 0<y2<1, 0<y3<1, 0.26x1+37y1<=1, 0.26x2+37y2<=1,0.26x3+37y3<=1,Eg_(GaN)(1−x1)(1−y1)+Eg_(GaAs)(1−x1)y1+Eg_(AlN)x1(1−y1)+Eg_(AlAs)x1y1>Eg_(GaN)(1−x2)(1−y2)+Eg_(GaAs)(1−x2)y2+Eg_(AlN)x2(1−y2)+Eg_(AlAs)x2y2,andEg_(GaN)(1−x3)(1−y3)+Eg_(GaAs)(1−x3)y3+Eg_(AlN)x3(1−y3)+Eg_(AlAs)x3y3>Eg_(GaN)(1−x2)(1−y2)+Eg_(GaAs)(1−x2)y2+Eg_(AlN)x2(1−y2)+Eg_(AlAs)x2y2,where Eg_(GaN), Eg_(GaAs), Eg_(AlN), and Eg_(AlAs) are the band gapenergy of GaN, GaAs, AlN, and AlAs, respectively.
 9. A light emittingdevice according to claim 8, wherein said active layer is a GaAlNAssingle or multiple quantum well active layer where Al content, xw, andAs content, yw of all the constituent layers satisfy the relationship of0<xw<1, 0<yw<1, and 0.26xw+37yw<=1.
 10. A light emitting deviceaccording to claim 8, wherein the condition of3.18(1−xs)(1−ys)+3.99(1−xs)ys+3.11xs(1−ys)+4xsys nearly equals aconstant value is satisfied, where xs and ys are the Al content and theAs content, respectively in each constituent layers.
 11. A lightemitting device according to claim 8, wherein the relationship of3.18(1−xs)(1−ys)+3.99(1−xs)ys+3.11xs(1−ys)+4xsys nearly equals 3.18 issatisfied, where xs and ys are the Al content and the As content,respectively in each constituent layers.
 12. A light emitting devicecomprising: a certain conduction type of aGa_(1−x1)Al_(x1)N_(1−y1)As_(y1) first cladding layer, aGa_(1−x2)Al_(x2)N_(1−y2)As_(y2) active layer, an opposite conductiontype of a Ga_(1−x3)Al_(x3)N_(1−y3)As_(y3) second cladding layer, saidGa_(1−x3)Al_(x3)N_(1−y3)As_(y3) second cladding layer has a ridgestructure, all successively formed one upon each other, wherein x1, x2,and x3 define the Al content, y1, y2, and y3 define the As content, andx1, y1, x2, y2, x3, and y3 have a relationship of 0<x1<1, 0<x2<1,0<x3<1, 0<y1<1, 0<y2<1, 0<y3<1, 0.26x1+37y1<=1, 0.26x2+37y2<=1,0.26x3+37y3<=1,Eg_(GaN)(1−x1)(1−yl)+Eg_(GaAs)(1−x1)y1+Eg_(AlN)x1(1−y1)+Eg_(AlAs)x1y1>Eg_(GaN)(1−x2)(1−y2)+Eg_(GaAs)(1−x2)y2+Eg_(AlN)x2(1−y2)+Eg_(AlAs)x2y2,andEg_(GaN)(1−x3)(1−y3)+Eg_(GaAs)(1−x3)y3+Eg_(AlN)x3(1−y3)+Eg_(AlAs)x3y3>Eg_(GaN)(1−x2)(1−y2)+Eg_(GaAs)(1−x2)y2+Eg_(AlN)x2(1−y2)+Eg_(AlAs)x2y2,where Eg_(GaN), Eg_(GaAs), Eg_(AlN), and Eg_(AlAs) are the band gapenergy of GaN, GaAs, AlN, and AlAs, respectively.
 13. A light emittingdevice according to claim 12, wherein said GaAlNAs active layer is aGaAlNAs single or multiple quantum well active layer where Al content,xw, and As content, yw of all the constituent layers satisfy therelationship of 0<xw<1, 0<yw<1, and 0.26xw+37yw<=1.
 14. A light emittingdevice according to claim 12, wherein the condition of3.18(1−xs)(1−ys)+3.99(1−xs)ys+3.11xs(1−ys)+4xsys nearly equals aconstant value is satisfied, where xs and ys are the Al content and theAs content, respectively in each constituent layers.
 15. A lightemitting device according to claim 12, wherein the relationship of3.18(1−xs)(1−ys)+3.99(1−xs)ys+3.11xs(1−ys)+4xsys nearly equals 3.18 issatisfied, where xs and ys are the Al content and the As content,respectively in each constituent layers.